Synchronous machines with permanent magnet excitation controlled by a variable-speed frequency converter and that can operate as a motor or as a generator, present very high power density per unit volume.
Machines of that type generate losses that are very low in the rotor and they are characterized by low negative stiffness. Such electrical machines are therefore well adapted for use in combination with magnetic bearings.
Such synchronous machines can operate at very high speed.
In one particular technology, the rotor of the synchronous machine comprises permanent magnets bonded to the surface of a solid shaft of magnetic steel. To avoid separation due to centrifugal forces, use is made of a binding band of carbon fibers or glass fibers. That technology is compatible with linear speeds that may be as great as 300 meters per second (m/s).
For power levels above about 50 kilowatts (kW), the stator of the machine is generally cooled by a liquid. Air or some other cooling gas can also be introduced into the airgap, thereby ventilating the airgap, but circumstances nevertheless exist in which it is difficult to cool such a machine.
Synchronous machines with permanent magnet excitation have a rotor without a damper cage. It is therefore necessary for them to be controlled at variable speed by a frequency converter. The converter feeds the stator coils of the machine with currents that are variable in amplitude and in frequency.
As mentioned above, it is possible to operate in motor mode or in generator mode. Variable-speed control in an open loop—similar to controlling a synchronous motor—further requires electronic stabilization of angular oscillations in order to avoid any risk of losing synchronization.
In order to improve the robustness of the system, it is preferable to use information concerning the angular position of the machine in order to synchronize the converter. Such external synchronization enables a rotating machine to be resynchronized automatically in the event of synchronization being lost.
Because of the way it operates, the frequency converter generates harmonics in the phase currents of the machine. These harmonics, and also direct current (DC), produce additional losses in the stator and the rotor. Rotor losses which appear under the binding band, in the magnets and in the solid shaft, are particularly critical in this type of machine. The composite binding band is a thermal insulator which prevents the rotor from cooling effectively. Even low levels of loss can lead rapidly to temperatures that are above the acceptable limit, which is situated at about 150° C.
To remedy that problem, proposals have already been made to use power filters for reducing harmonic content to below the acceptable value. For a machine with power greater than 50 kW, the order of magnitude for a maximum acceptable threshold corresponds to the total harmonic content being about 5% to 10%.
Even when using power filters, it can happen in practice that harmonic distortion increases to above the maximum acceptable value, e.g. because of instability in the electronic circuits controlling the frequency converter, or indeed because of a failure in a power filter or in the interconnections.
Under such circumstances, the temperature of the rotor increases very quickly, running the risk of destroying the binding band for holding on the permanent magnet, and then to the entire machine being destroyed.
Conventional solutions, e.g. monitoring the temperature of the stator, do not enable such localized heating to be avoided, particularly when it takes place in the binding bands.